Fix S1168 FP: Does not respect nullable annotations

[nesc58]

Description

The rule S1168 raises false positive when then nullable annotation is enabled.

Repro steps

public interface IInterface
{
    public byte[]? GetNullableFileContent(string itemId);
    public byte[] GetFileContent(string itemId);
}

public class Implementation : IInterface
{
    public byte[]? GetNullableFileContent(string itemId) => null; // Should not raise because nullable is allowed
    public byte[] GetFileContent(string itemId) => null; // Should raise because null is unexpected
}

Expected behavior

Should not raise when nullable is enabled and the syntax is used.
For me there is a big difference between an empty list and a not-existing list. Those are two completely different statements.
On the one hand, not existing means that there is no information about it. On the other hand, an empty list means that, for example, no information has been entered.

For example:
Returning null means "there is no file". Returning an empty array means "there is a file but this file is empty".

I mean rule is very useful when it is required that the value is notnull. When the annotation allows nullable values it is very annoying because the nullables allows the difference between "there is nothing" and "there is something empty".

I think this rule should not be triggered when nullable is enabled and used because the compiler (on newer versions) warns when not checking a nullable value

Actual behavior

Rule will be raised when nullable is enabled and used

[mary-georgiou-sonarsource]

Thank you for opening this issue, @nesc58. Your feedback helps us improve our products.

I confirm this is an FP.
I added the issue to our backlog to be handled in one of our next sprints.

Best regards
Mary